Engine systems may use humidity sensors to monitor ambient humidity levels in order characterize engine operating conditions and appropriately control engine parameters such as fuel injection, valve and ignition timing, and boost pressure to increase performance and fuel efficiency and reduce emissions. Some engine systems may also rely on the output of a humidity sensor positioned in an intake manifold to control exhaust gas recirculation (EGR) from an engine exhaust system to an engine intake system, to reduce emissions, among other purposes. In one example, in addition to adjusting EGR based on engine speed and engine load, these humidity sensors may also provide input data for controlling an EGR valve to adjust the amount of recirculated exhaust gas flow and achieve a desired intake air dilution based on engine operating conditions, thus maintaining desirable combustion stability. Recirculated, and in some instances, cooled exhaust gas may be combined with fresh intake air drawn into the intake passage resulting in a mixture of fresh intake air and recirculated exhaust gas entering the engine. Thus, an estimate of EGR flow may be determined based on an output of the humidity sensor positioned in the intake or intake manifold, and therefore be used to control EGR flow to a desired level based on additional engine operating conditions. If the humidity sensor is not functioning correctly, an inaccurate EGR flow estimate may be determined, thereby resulting in controlling the EGR flow rate to a rate different than desired, which may result in increased emissions and/or reduced engine performance.
Because humidity sensor output data affects engine performance, fuel efficiency, and emissions, a robust intake manifold humidity sensor diagnostic is needed. Other attempts to provide an intake manifold humidity sensor diagnostic include using a condensation heater and temperature sensor coupled to the humidity sensor to increase the local temperature and thus the corresponding humidity proximate the humidity sensor to invoke a humidity sensor response. One example approach is shown by Pursifull, et al. in U.S. Pat. No. 9,329,160. Therein, Pursifull aims to model the relative humidity as a function of pressure and temperature for a given absolute humidity. By comparing the modeled output of the humidity sensor to the actual output of the humidity sensor responsive to actuation of the condensation heater, degradation may be determined if the difference between the modeled output and the actual output surpasses a margin of error threshold.
Other attempts to provide robust methods for diagnosing an intake manifold humidity sensor include selectively diagnosing a fault in the humidity sensor based on a comparison between a first humidity value from a humidity sensor in the intake manifold and a second humidity value based on humidity data from another source. One example approach is shown by Bauerle in U.S. Pat. No. 8,315,759. Therein, Bauerle demonstrates receiving a signal from a data source external to the vehicle, such as a wi-fi or television signal to determine a baseline indication of ambient humidity. Other example approaches include comparing the output of a humidity sensor positioned in the intake manifold to the output of a humidity sensor positioned in a passenger compartment of the vehicle, or to an estimation of humidity based on an oxygen sensor positioned in an exhaust passage of the engine.
However, the inventors herein have recognized potential issues with such systems. As one example, coupling a condensation heater and a temperature sensor to a humidity sensor for the purpose of verifying the humidity sensor's functionality increases the cost of production and the number of components to be housed in an engine compartment where space is already in short supply. Further, the method relies on affecting humidity sensor output using the condensation heater, and modeling humidity sensor data based on output from the temperature sensor, both of which may themselves suffer degradation. In other examples, rationalizing humidity sensor functionality against a separate and/or distant humidity sensor may lead to error as the humidity may be different at the respective sensor locations, making comparison of sensor output moot. Additionally, many noise factors exist which may cause incorrect diagnoses when comparing two different sensor outputs. In the example of using an oxygen sensor in the exhaust system to estimate humidity, incomplete combustion, the temperature of the exhaust, and intake manifold leaks may skew sensor output and give a faulty reading. Furthermore, some engines, such as start/stop engine and PHEVs may have limited engine run time, and using downstream sensors to diagnose humidity sensor function may not present sufficient opportunities for diagnosis. Lastly, some regions are naturally dry and arid, which may not readily provide the humid conditions suitable for regular evaluation of the humidity sensor.
In one example, the issues described above may be addressed by a method for an engine, comprising: after an engine-off duration: combusting fuel at cylinders of the engine while flowing gases through the engine in a first direction; switching to flowing gases through the engine in an opposite, second direction while not combusting fuel; during the flowing gases in the second direction, obtaining an output of a humidity sensor positioned in an engine intake; and indicating degradation of the humidity sensor based on the output. In this way, an increased and/or known quantity of water vapor generated by combustion may be drawn from the exhaust, through the combustion chambers, and past the humidity sensor positioned in the intake manifold of the engine. By introducing an increased and/or known quantity of water vapor, a given humidity sensor response may be expected.
As one example, degradation of the humidity sensor may be indicated responsive to the output of the humidity sensor not changing by a threshold amount between flowing gases through the engine in the first direction and flowing gases through the engine in the opposite, second direction. Further, in response to indicating degradation, subsequent engine operation may not be based on the output of the humidity sensor (e.g., EGR flow may not be estimated and/or controlled based on the output of the degraded humidity sensor). In this way, by monitoring the humidity sensor response while flowing gases through the engine in the first (e.g., forward) and then second (e.g., reversed) directions, an expected humidity sensor response may be characterized, and degradation may be diagnosed without relying on remote humidity sensor indications, or the use of supplemental heaters and temperature sensors.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.